A CMOS (Complementary Metal-Oxide Semiconductor) image sensor may receive an external light, convert the light into an electrical signal, and output the electrical signal. As being a detection circuit of a CMOS image sensor, an Active Pixel Sensor (APS) circuit has a non-uniform outputting current during a photoelectric conversion process of a photosensitive device, since the process of the source follower thin film transistors (TFT) may have a difference. Thus, the outputting current of the source follower TFT will be affected by its own threshold voltage, causing a display distortion.